Sensor having an offset voltage and method of operation

ABSTRACT

A diaphragm (30) flexes in response to varying forces applied to the diaphragm (30). This motion is monitored by a transducer (31) that is preferably in a Wheatstone bridge configuration. When the diaphragm (30) is in a relaxed condition, with little or no force applied to the diaphragm, an offset voltage is generated by highly doped contact regions (35,36,37,38). At least one of these highly doped contact regions (35,36,37,38) is configured to have more squares of material which increases the resistance of that particular highly doped contact region (37).

BACKGROUND OF THE INVENTION

This invention relates, in general, to semiconductor devices, and moreparticularly, to sensors and methods of operation.

Semiconductor devices, such as piezoresistive pressure sensors, canutilize a thin semiconductor layer as a sensing diaphragm to detect apressure differential across the diaphragm. To determine the approximatevalue of the pressure differential, a transducer comprising a Wheatstonebridge is used to measure the strain in the diaphragm. The Wheatstonebridge is made of piezoresistors whose resistance varies with thedeflection of the diaphragm. As the diaphragm flexes with changingpressure, the transducer generates a voltage potential between twooutput pads. Traditionally, when there is no pressure differentialacross the diaphragm, there is no voltage potential present between thetwo output pads.

Some prior applications require that either a positive or negativevoltage potential be present across the output pads when there is nopressure differential across the diaphragm. To generate this offsetvoltage, the previously known transducer structures have relied onadjusting or skewing the configuration of the Wheatstone bridge so thateach electrical path has a net difference in resistance value. In doingthis, these methods adjust the relative length or width of each leg inthe network and move the placement of contact points across theWheatstone bridge. Although this is a minor structural modification, itinvolves changes not only in the manufacturing process of the sensor,but changes in the performance and characteristics of the sensor aswell.

For example, to change the relative location of the piezoresistors inthe Wheatstone bridge requires that two of the photolithographic masksbe changed. The first mask moves the location and size of thepiezoresistors and the second mask adjusts the location of the contactregions to the Wheatstone bridge since the location of thepiezoresistors has changed. From a manufacturing standpoint, thechanging of two photolithographic masks to meet the requirements of asingle customer's request is expensive. Since a particular offsetvoltage requires a change in masks at two separate process steps, it isnot possible to have a manufacturing flow that allows material to bestaged at a convenient point in the process flow. The change in twomasking layers also hinders the throughput of the process flow andlimits the flexibility of a manufacturer to varying customer demands.All of these limitations ultimately increase the final manufacturingcost of the sensor product.

In addition to complicating the manufacturing flow, skewing the shape ofthe Wheatstone bridge also affects the performance of the sensorcomponent. The accuracy, temperature coefficient of offset (TCO),linearity, and impedance of the sensor are all strong functions of theconfiguration of the Wheatstone bridge. Any skewing that is done to theconfiguration will generate an offset voltage across the output pads,but it is accomplished at the cost of reducing the linearity andaccuracy of the sensor. Again from a manufacturing standpoint, ascustomers request various offset voltages, each configuration of theWheatstone bridge must first be tested to determine the impact it willhave on the performance of the sensor. This puts the manufacturer in adifficult position in that it must not only produce a sensor that hasthe particular offset voltage required by the customer, but it stillmust meet the customer's requirements for accuracy, TCO, and linearity.

By now it should be appreciated that it would be advantageous to providea method for adjusting an offset voltage of a sensor without requiringthe modification of two photolithographic masks. It would even be moreadvantageous if the method could adjust the offset voltage withouthaving any significant impact on the linearity or accuracy of thesensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged top view of a previously known sensor;

FIG. 2 is an enlarged top view of a previously known sensor in analternate skewed configuration;

FIG. 3 is an enlarged top view of a sensor according to the presentinvention; and

FIG. 4 is an enlarged top view of a portion of the sensor in FIG. 3demonstrating an alternate configuration.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning to the figures for a more detailed description of the presentinvention, FIG. 1 is an enlarged top view of a portion of a previouslyknown sensor 50. Sensor 50 comprises a diaphragm 10 that moves inresponse to changes in a pressure differential across diaphragm 10. Thisresponse is measured by a transducer 11, which is made up ofpiezoresistors 12 arranged as a Wheatstone bridge. Electrical contact ismade to transducer 11 at contact points 13 by highly doped contactregions 15, 16, 17, and 18. These highly doped contact regions 15-18 areused to apply a voltage potential across two contact points 13 oftransducer 11 and are used to measure the voltage potential present atthe other two contact points 13. Each of these voltage potentials iseither provided by or measured by external electrical contact usingmetal lines 20 and reference pads or output pads 21, 22, 23, and 24.

For example if a 5 volt voltage potential is placed across output pads21 and 23, there will be a 5 volt voltage potential across transducer 11at contact points 13 made by highly doped contact regions 15 and 18. Asdiaphragm 10 flexes, the resulting strain will change the resistancevalues of piezoresistors 12 which will result in a difference in voltagepotential between highly doped contact regions 17 and 16. This voltagedifferential can be measured at output pads 22 and 24 and be used tocalculate the relative pressure differential across diaphragm 10. Ifdiaphragm 10 is in a relaxed condition and there is no pressuredifferential across diaphragm 10, there will be no force applied totransducer 11. This traditionally means that there is no voltagedifferential between output pads 22 and 24.

Some customer applications, however, are more efficient if transducer 11can produce a voltage differential, referred to as an offset voltage,when diaphragm 10 is in a relaxed state. In order to generate an offsetvoltage when diaphragm 10 is in a relaxed condition, some previouslyknown sensors skew the configuration of transducer 11 as shown in FIG.2. FIG. 2 is an enlarged top view of previously known sensor 50 wheretransducer 11 is formed by skewing the configuration of piezoresistors12 and 14. As shown in FIG. 2, transducer 11 is made from piezoresistorshaving two different widths (i.e. piezoresistors 12 are thinner thanpiezoresistors 14.) As a result, piezoresistors 12 will have a higherresistance value and produce a lower voltage potential at contact point13 made with highly doped contact region 17. Therefore, when diaphragm10 is in a relaxed condition, there will be a natural offset voltagepotential between output pads 22 and 24. This offset voltage istypically on the order of about -20 millivolts to 20 millivolts.

As discussed in the background, this technique of generating an offsetvoltage not only complicates the manufacturing process of making thesensor, but it can dramatically affect the performance of the sensor aswell. Physically, to skew the configuration of transducer 11 requireschanges to both the photolithographic mask used to patternpiezoresistors 12 and 14 and the mask used to pattern highly dopedcontact regions 15-18. In order to offer customers a variety ofdifferent offset voltages, each separate voltage would require a new setof expensive masks which presents logistical problems in keeping trackof all the different configurations.

An even bigger problem with skewing the configuration of transducer 11,is the impact it has on the accuracy, TCO, and linearity of the sensor.The performance of a sensor is a significant function of how diaphragm10 applies a force to transducer 11. By adjusting the physical layout oftransducer 11, the amount and profile of the strain applied to eachpiezoresistor 12 and 14 is also changed. Each configuration oftransducer 11 would require thorough characterization to understand theimpact the configuration had on the performance of the sensor.

Turning now to FIG. 3, an improved method for forming a sensor having anatural offset voltage will be provided. FIG. 3 is an enlarged top viewof a diaphragm 30 of a sensor 51 according to the present invention.Diaphragm 30 can be part of a sensor or part of a semiconductor deviceused in a variety of applications. Such sensors include, but are notlimited to, a pressure sensor, an accelerometer, a gyro, and a chemicalsensor. To measure the force or strain in diaphragm 30, a transducer 31is used to generate an electrical response to the motion of diaphragm30.

Preferably, transducer 31 is a configuration of piezoresistors 32arranged as a Wheatstone bridge. It should also be appreciated thattransducer 31 can be any structure capable of measuring the deflectionin diaphragm 30 and can comprise piezocapacitors or be made from apiezoelectric material. Electrical contact is made to transducer 31 atcontact points 33 to measure changes in voltage potential or currentflow as diaphragm 30 moves.

The response of transducer 31 is communicated by highly doped contactregions 35, 36, 37, and 38 that are coupled to contact points 33. Unlikepreviously known configurations, at least one of the highly dopedcontact regions 35-38 is configured to adjust the resistance of thisparticular path. As shown in FIG. 3, highly doped contact region 37 isconfigured in a "c" pattern so that it has more squares of resistivematerial than highly doped contact regions 35, 36, or 38. Highly dopedcontact region 37 has a first region that makes contact to transducer 31and a second region that provides the resistive path that generates anoffset voltage. The term `squares` is an industry term used by those wholay out the configuration of semiconductor devices. It provides adimensionless and quantitative measurement of the relative surface areaof a structure compared to other structures in the layout. Simplystated, if a structure has a width `W,` then its length `L` divided by`W` is the number of squares of that material.

Highly doped contact regions 35, 36, 37, and 38 are electrically coupledto external systems by reference pads or output pads 41, 42, 43, and 44respectively. As shown in FIG. 3, this coupling is provided by metallines 40. Since highly doped contact region 37 has more squares ofsemiconductor material than highly doped contact regions 35, 36, and 38,it will provide a more resistive path and provide a lower voltagepotential on output pad 42. Therefore, a voltage potential differentialof about -20 millivolts to 20 millivolts will be present between outputpads 42 and 44 which provides the offset voltage when diaphragm 30 is inrelaxed condition.

An experiment was performed which compared the offset voltage generatedby previously known sensor 50 of FIG. 1 to sensor 51 according to thepresent invention as shown in FIG. 3. In each case, there was no stressapplied to the diaphragm. The previously known sensor generated anoffset voltage of -6.7 millivolts and a sensor according to the presentinvention generated an offset voltage of +16.6 millivolts. The presentinvention, therefore, can be used to adjust an offset voltage comparedto a standard transducer without having to adjust the configuration ofthe transducer used to monitor the diaphragm.

It should also be understood that highly doped contact region 37 canhave a variety of configurations used to adjust the resistance value ofthis electrical leg. For example, FIG. 4 shows highly doped contactregion 37 arranged in a serpentine pattern. Contact region 37 isconfigured so that its resistive value is not symmetrical to theelectrical path of other reference voltage connections. Preferably,contact region 37 is configured to have 3 squares to 300 squares ofsemiconductor material which provides the resistance value necessary togenerate the offset voltage.

A method for forming the highly doped contact regions 35-38 of thepresent invention will now be provided. After forming transducer 31 ondiaphragm 30, highly doped contact regions 35-38 are formed by doping aportion of the sensor. Preferably, highly doped contact regions 35-38are made by patterning diaphragm 30 or a silicon substrate using aphotolithographic mask and then implanting the semiconductor materialwith either a p-type or n-type species. For example, the semiconductormaterial can be implanted with a boron source at a dose of 1E12atoms/cm² to 1E15 atoms/cm² with an energy of 50 keV to 100 keV. Thismethod can be used to form highly doped contact regions 35-38 with asheet resistance of 1 ohm/sq to 1 kilo ohm/sq. Metal lines 40 andreference pads 41-44 can then be formed by patterning a conventionalconductive layer as used in the industry. This will allow an externalsystem to perform analysis of the response signals generated bytransducer 31.

Adjusting the resistance value of highly doped contact region 37, bychanging its configuration, does not affect the accuracy or linearity ofthe response generated by transducer 31. The present invention,therefore, provides a method for adjusting the offset voltage generatedby a sensor, which does not involve skewing the configuration of thetransducer that monitors the diaphragm. The present invention is a muchmore manufacturable method since different offset voltages can begenerated without having to re-evaluate the impact each change has onthe performance of the sensor. The present invention, therefore, allowsthe optimal configuration of the Wheatstone bridge to be used, since itsconfiguration need not be adjusted to generate an offset voltage.

The present invention also provides a method that is a much more costeffective solution since only one photolithographic mask needs to bechanged to adjust the number of squares in highly doped contact region37. This allows a manufacturing facility to stage material at one pointin the process flow and only vary one photolithographic step to producesensors with various offset voltages. This improves the throughput inthe manufacturing facility and reduces the cost of each sensor produced.

By now it should be appreciated that the present invention provides asensor, and a method of operation, that has an offset voltage when adiaphragm is in a relaxed condition. This eliminates all the cost andperformance issues associated with skewing the configuration of atransducer. The present invention provides a method for adjusting theoffset voltage of the sensor that is cheaper and easier to implement ina manufacturing facility.

I claim:
 1. A sensor having an offset voltage comprising:a diaphragmthat flexes in response to a pressure differential across the diaphragm;a transducer comprising a plurality of legs that are in physical contactwith each other, wherein the transducer is coupled to the diaphragm fordetecting the pressure differential and generating an electricalresponse to the pressure differential; at least two reference padselectrically coupled to the transducer; and a resistive path between thetransducer and one of the at least two reference pads so that the atleast two reference pads provide an offset voltage, wherein theresistive path is adjacent to the transducer.
 2. The sensor having anoffset voltage of claim 1 wherein the offset voltage is about -20millivolts to 20 millivolts.
 3. The sensor having an offset voltage ofclaim 1 wherein the resistive path comprises a semiconductor materialarranged to provide a resistive path between the transducer and one ofthe at least two reference pads.
 4. The sensor having an offset voltageof claim 3 wherein the semiconductor material comprises 3 squares to 300squares of semiconductor material.
 5. The sensor having an offsetvoltage of claim 3 wherein the semiconductor material is doped to asheet resistance of 1 ohm/sq to 1 kilo ohm/sq.
 6. The sensor having anoffset voltage of claim 3 wherein the semiconductor material comprises aplurality of squares arranged in a "c" pattern.
 7. The sensor having anoffset voltage of claim 3 wherein the semiconductor material comprises aplurality of squares arranged in a serpentine pattern.
 8. The sensorhaving an offset voltage of claim 1 wherein the transducer is aWhetstone bridge.
 9. The sensor having an offset voltage of claim 1wherein the transducer comprises at least one piezoresistor.
 10. Thesensor having an offset voltage of claim 1 wherein the sensor is asensor selected from the group consisting of a pressure sensor, anaccelerometer, a gyro, and a chemical sensor.
 11. A sensor comprising:adiaphragm having an edge; a transducer having four legs that aresubstantially equal in size, in physical contact with each other, andcoupled to the diaphragm, wherein a portion of the transducer isoverlying the edge of the diaphragm; at least two reference pads coupledto the transducer; and a resistive path having a plurality of squaresbetween the transducer and one of the at least two reference pads,wherein a portion of the resistive path is coincident with the edge ofthe diaphragm and provides an offset voltage.
 12. A semiconductor devicecomprising:a diaphragm that flexes; a Wheatstone bridge comprising aplurality of piezoresistors, wherein each of the plurality ofpiezoresistors is substantially equal in size, are in physical contactwith adjacent piezoresistors of the plurality, and have a contact pointthat provides a voltage potential as the diaphragm flexes and an offsetvoltage when the diaphragm is relaxed; and a contact region coupled toat least one contact point, the contact region comprising asemiconductor material with a first region and a second region, thefirst region coupled to the Wheatstone bridge and the second regionbeing coupled to the first region and having a plurality of squares. 13.The semiconductor device of claim 12 further comprising reference padscoupled to the Wheatstone bridge, wherein the second region of thecontact region generates an offset voltage across the reference pads.14. The semiconductor device of claim 13 wherein the offset voltage isabout -20 millivolts to 20 millivolts.
 15. The semiconductor device ofclaim 12 wherein the semiconductor material is doped to a sheetresistance of 1 ohm/sq to 1 kilo ohm/sq.
 16. The semiconductor device ofclaim 12 wherein the plurality of squares are arranged in a "c" pattern.17. The semiconductor device of claim 12 wherein the plurality ofsquares are arranged in a serpentine pattern.
 18. The semiconductordevice of claim 12 wherein the semiconductor material comprises silicon.19. The semiconductor device of claim 12 wherein the semiconductordevice is a sensor selected from the group consisting of a pressuresensor, an accelerometer, a gyro, and a chemical sensor.
 20. A methodfor generating an offset voltage comprising the steps of:providing atransducer having a plurality of legs, wherein adjacent legs of theplurality are in physical contact with each other; applying a force to adiaphragm which flexes in response to the force; sensing motion of thediaphragm with the transducer; and conducting a response from thetransducer with a semiconductor material having a plurality of squaresthat generates an offset voltage when the diaphragm is in a relaxedcondition, the semiconductor material being coupled to the transducer.21. The method for generating an offset voltage of claim 20 wherein thestep of applying the force includes applying a differential pressureacross the diaphragm.
 22. The method for generating an offset voltage ofclaim 20 wherein the step of sensing motion of the diaphragm isperformed with a Wheatstone bridge.
 23. The method for generating anoffset voltage of claim 20 wherein the step of sensing motion of thediaphragm is performed with at least one piezoresistor.
 24. The methodfor generating an offset voltage of claim 20 wherein the step ofconducting the response from the transducer includes passing theresponse to an output pad, the output pad having an offset voltagecompared to a reference pad.
 25. The method for generating an offsetvoltage of claim 20 wherein the step of applying a force to thediaphragm includes providing a diaphragm that is part of a sensorselected from the group consisting of a pressure sensor, anaccelerometer, a gyro, and a chemical sensor.